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				   why only concentric rings are seen on the screen and how u conclude that these are not because of X-rays

7 years ago


Answers : (1)


Dear student,

About the same time as Davisson and Germer, Sir G.P. Thompson, working with his assistant A. Reid, took a thin gold foil, rolled it to be extremely flat, placed it in front of a 30 kV electron beam, and recorded the accompanying image on their photographic plate. The rings are associated with electron diffraction from the interplanar spacing within tiny crystallites in the gold foil. The continuous rings arise because the tiny crytallites are randomly oriented in the foil. Subsequently they took a sample of aluminium and did not roll it but worked carefully with it to obtain a crystalline sample. When this was exposed to their beam, spots lying around a ring-like structure were observed. This is seen in the figure below. Electrons indeed behave exactly as if they were waves.
Thompson's Aluminum Crystal Experiment

It is interesting to recall that G.P. Thompson, who shared the 1937 Nobel Prize with Davisson for these experiments which proved that electrons are waves, is the son of J.J. Thompson who received the Nobel Prize in 1906 for proving that cathode rays were actually particles - electrons! And the amazing thing is that they were both right.





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